1. Field of the Invention
The invention relates to a device for carrying out a plasma enhanced process, in particular a plasma enhanced chemical vapour deposition. The device serves e.g. for coating one side of a web of film or sheet material, in particular for coating a web of polymer film with silicon oxide in order to improve its barrier properties.
2. Description of Related Art
It is known e.g. from the publication EP-299754 (BOC) to deposit a thin film of silicon oxide on a substrate in a plasma enhanced chemical vapour deposition process. According to this publication a vacuum chamber is provided in which the electrically isolated substrate is positioned to face a magnetron being powered with an alternating voltage. A stream of a process gas mixture consisting of an organosilicon compound, oxygen and an inert gas (e.g. argon or helium) is flown into the space between the face of the magnetron and the substrate and the plasma derived from the process gas mixture is maintained at a pressure of e.g. 6 Pa.
The magnetron described in the publication EP-0299754 is a flat magnetron of a balanced or unbalanced type. The degree of unbalance of a flat magnetron depends on the strength ratio of the magnetic pole or poles positioned on either side of the track running around the magnetron face, i.e. on the ratio between the number of magnetic field lines extending from north to south poles across the track to the number of field lines not doing so. The unbalanced magnetron is known to not fully confine electrons and ions of the plasma such that there is a limited amount of electron and ion bombardment of the substrate which is said to improve the deposition quality. The magnetron face of the unbalanced magnetron according to EP-0299754 comprises peripheral north poles and a central core of soft iron, resulting in an only small portion of the field lines extending from the north poles to the core (highly unbalanced magnetron).
Publication EP-0605534 (BOC) describes a similar plasma enhanced chemical vapour deposition process wherein the substrate is a web. The web is carried by a rotating drum which constitutes the powered electrode and is negatively biased. Facing the web carried by the drum there is an electrically grounded and possibly cooled shield the back side of which is faced by at least one pair of opposing magnetic poles, preferably a series of alternating magnetic poles. Aided by the magnetic field the plasma is confined between the drum and the shield. The advantage of the arrangement is seen in the decoupling of the electric and the magnetic field, which is said to lead to an extension of the plasma throughout the plasma volume (between drum and shield). The distance between drum and shield is described as having to be within the range of 1 to 30 cm.